WHAT DOES decrease 1 st at each end of next and every alt row mean?

Does this instruction mean I knit 2 stitches together at row 1 and 3 and 5, etc…?

Yes.

at EACH end of row 1, 3, 5, etc. So you’ll k2tog the first two stitches AND the last two stitches of each of those rows.

what if is p1 k1 pattern, I did slip 1 and repeat to decrease at end of row, is this ok

Yes, you can decrease after the slip 1 and before the last stitch.