Theres a number of DNS seed servers that are hard coded into the Bitcoin core codebase. A Bitcoin client does a nslookup on these seeds to find intial peers to connect to and bootstraps this way.

For a bit more detail and if you're interested in this sort of thing, have a read of my blog article I wrote about the technical side of Bitcoin: http://www.samlewis.me/2017/06/a-peek-under-bitcoins-hood/