Is it this one?
"Receive -> Request" will give you the "first" unused address from your wallet... it is probably the best way to get an address to "send yourself coins" (or to give to someone else so they can send you coins), unless you really know what you're doing and need/want to see a specific address from your wallet. I would recommend you use this method.
Or this one?....
This method is basically just showing you ALL your receiving addresses at once. it also indicates which ones are "used" (aka have received coins) and which ones are "unused".
Electrum is what is known as a "Hierarchical Deterministic" or "HD" Wallet... this means that it uses a "seed" (derived from the 12 words you would have been shown when you created the wallet) and from that seed it will be able to automatically generate an endless number of receiving addresses. Electrum, by default, pre-generates around 25 addresses. As they become "used" (ie. they receive coins), the wallet will generate new "unused" addresses. This list shows all the ones that it has currently generated.
The reason to use a new address for each receive transaction is to prevent/reduce "Address re-use", which can reduce privacy of both the sender and receiver. It's not really a security issue. You can read more about that topic here:
https://en.bitcoin.it/wiki/Address_reuseAnd what does "change" (circled in red) do?
The "Change" button, just switches the list from receiving addresses to "change" addresses (You don't really need to worry about change addresses, Electrum will use them automatically and you should not use them as an address to receive coins from an external source.)