Broadly speaking population bottleneck can mean either the number of people on the earth falls drastically just like our bank balance at the end of the month or the genetic variety of the population falls that is we would be left with the same type of dollar notes. This genetic bottleneck can either be good or bad. It's bad if you are only left with $1 bills or in human terms; idiots who can't count till ten. Or it can be a good thing whereby you have all $100 bills or in human terms Phd scholars from MIT who are also Olympic gold medalists cum Billionaires.
There are various population bottlenecks humanity has been unfortunate enough to come across.
One is a Toba Catastrophy theory:
Presented sometime around the start of the millennium, it proposed that a Toba supervolcano in Indonesia lost its patience and royally erupted sometime 70,000 years ago. The resulting aftermath was so devastating that only about 10,000 to 30,000 people managed to get into anti-volcano bunkers and save their lives. This conclusion was drawn upon on the basis of sudden climate change like the way one feels after they step out of an Ac room in 100F and low level of genetic variation amongst humans in that period. All went well for a decade or so before new evidence came out to suggest that neither the climate change nor the genetic bottleneck was intense enough to cause such a decimation of the human race.
Another population bottleneck seems to have cropped up about 14,000 years ago when humans first inhabited North America. As claimed by a study just 70 people crossed over to the continent via a land bridge. Yes, when the whole world was refrigerator during the ice age there was a land bridge probably made up of ice strong enough for one to drive a corvette and just 70 people crossed over and became the first human inhabitants of a land. So if you just have the genes of 70 people at your disposal you would undoubtedly have the mother of all population bottlenecks but somehow a meteor didn't strike or an elk didn't ground them to dust and they thrived.
One of the most well known population bottlenecks occurred 50,000 years ago when humans finally got bored of staying in Africa and started backpacking to other continents. Further, they went from Africa the more they lost their genetic diversity and their genetics kept becoming linear. It might have been a result of either a single bottleneck or a string of bottlenecks.
Besides these three, humans faced a different kind of a population bottleneck some 7000 odd years ago. Humans were getting tired of being clueless barbarians going out every night to hunt for dinner and had started simpler and less deadly activities like farming. So as this transition took place, for the first time in human history men started finding it difficult to get dates. It was noticed that during this period one male was responsible for reproduction with as many as 17 females that is kids born to 17 mothers had the same father. And you though 21st century was odd. So in simple terms, while most of the men were single desperately looking for someone a fortunate few were able to go about with as many as 17 females. Such a form of sexual capitalism ensured that 1% of the male population had access to an exorbitantly high amount of female resources. Either this happened because an unusually amount of males lacked social skills or that humans first saw a powerful elite class emerging who had access to most of the wealth aka tools of survival. That is as the human race wasn't yet sophisticated enough to ensure either a police force or social security females looked liked gyrating towards these elite males who were the difference between a comfortable life and ending up in a tiger's stomach. Or another explanation can be as homes started coming up most males became busy with Neolithic equivalents of Ps4 or football or basketball or Netflix and had no time for females!
This continued for some time but eventually, there seems to have been some kinda recession wherein the ancestors of Lehman Brothers, Meryl Lynch amongst others went bankrupt and couldn't afford 17 females which have ensured the ancestral ratio has come down to 4 females per 1 male.