A capitalization table, or cap table, tells you who owns how much of which types of stock in your company. Venture Hacks has an excellent educational blog post on how to make cap tables, together with a simplified template. Fred Wilson of Union Square Ventures has an equally excellent educational blog post with a second template for the cap table.
For a light hearted treatment of this very dry topic, read this highly entertaining 2017 TechCrunch article by Jason Rowley titled "Cap tables, share structures, valuations, oh my! A case study of early-stage funding. This is a parable about a fictional Silicon Valley startup called "Internet of Wings" and follows them through an initial seed round and a Series A raise.
Use these readings to learn the basics - and when the time comes to raise your first round of financing, work with a lawyer to construct your first cap table to ensure everything is in order.