come across (something)

This phrase means to find something without really trying to look for it:

But sometimes you'll be flipping through the channels and come across something really interesting.

Aren't these great? I came across them at a thrift shop a couple weeks ago.

a couple of (things)

"A couple" means two, or approximately two.

"A couple of weeks" means two weeks. But "a couple" is not as exact as "2", so you might say "a couple of weeks ago" if something happened recently but you don't remember exactly when.

Sometimes we leave out "of" in spoken English:

I'll be there in a couple minutes.

In writing, though, we almost always include "of".

Isn’t this (adjective)? / Aren’t these (adjective)?

You can use this structure to share your opinion or emotion with another person. 

Aren’t these cute?

Isn’t this weird?

This structure is very similar to saying, “This is (adjective), isn’t it?”

These are great, aren't they?

Although both are questions, they aren’t really open to all answers. With both structures, the speaker expects the listener to say yes. If you disagree, you can say so, but it may be considered impolite.

thrift shop

A “thrift shop” is a store for second hand (used) clothes and household items.