Here; this place (near the speaker).
ここが私の部屋です。
This is my room.
ここに座ってください。
Please sit here.
その箱はここに置いておいて。
Leave that box here.
At this point / now — referring to the current moment or situation.
ここで重要なのは準備です。
What matters here is preparation.
ここが問題の核心だ。
This is the core of the problem.
ここはもう諦めるしかないと思う。
I think at this point we just have to give up.
Used before a duration to mean 'these past ...' (e.g. ここ三年 = the past three years).
ここ三年で会社の方針が大きく変わった。
The company's policy has changed a lot over the past three years.
ここ数ヶ月は忙しくて休めなかった。
I was busy and couldn't rest for the past few months.
Used before a duration to mean 'the coming/next ...' or 'these next ...' (e.g. ここ一週間 = for the next week).
ここ一週間は出張が続きます。
I'll be on business trips for the next week.
ここ数日、天気が崩れるそうだ。
The weather is supposed to be bad for the next few days.