Normally, a remark is a comment. Your immediate reaction to what you see, hear, etc at that moment.
"I remarked how lovely his home is." or "I heard someone remark, 'he could have done better'."
One meaning of "mark" is "to notice". A "remark" is, as Rawdah said, actively noticing - usually by commenting. Sometimes you can remark quietly, eg. "I remarked to myself that the oven was dirty." You observed something, and decided it was significant.