this; this one — a demonstrative pronoun indicating something near the speaker or the current topic.
これ、ください。
This one, please.
これが私の財布です。
This is my wallet.
これを見て、どう思う？
Look at this—what do you think?
this person — used to indicate someone on the speaker's side or in-group (this one, this person).
これがうちの部長です。
This is our department manager.
これは彼の友達だよ。
This is his friend.
now; this point in time — used in temporal expressions (as in これから, これまで).
これから会議が始まります。
The meeting will start now / from now on.
これまでご協力ありがとうございました。
Thank you for your cooperation up to now.
(dated) here — an older/literary usage meaning 'here' (in place).
これにて儀を終える（古めの文体）。
With this, the ceremony ends. (older/formal style)
(formal/literary) used to emphasize the subject of a sentence; seen in formal or literary contexts.
これ、彼の主張は正当である（やや硬い表現）。
This—his claim is justified. (somewhat formal)
(archaic) I; me — an old-fashioned first-person pronoun (rarely used today).
これ、かの者に告げよ（古文調）。
I say—inform that person. (classical/archaic style)