It's not a stupid question. Fortunately, the answer is good news.
cs, ds, es, and on 386+, fs and gs are "segment registers" - different instructions from "general purpose registers". They are 16-bit registers, in all cases. In combination with an "offset", they form a complete address.
In real mode, the value in a segment register is multiplied by 16 (shifted left 4 bits) and added to an offset to form a complete address.
The CPU is switched to protected mode by setting bit 0 in the "control register", cr0. At this point, the rules change. The value in a segment register serves as an index into a table of descriptors. One of the fields in these descriptors is the "base", which is added to an offset to form a complete address. In any OS you're likely to encounter, the "base" is zero, so the offset is the address. There's an exception to this: fs is used for "thread local storage". Worry about that when you get to it.
Unless you're writing your own OS, the OS will have done this for you. You don't need to mess with segment registers - probably not even allowed to! Segment registers still exist, and they're still 16-bit registers, but (unless you're writing your own OS) you can forget that they exist. Trust me, that's "good news"!
Best,
Frank
