See the attached for assembly of the Zenvida 3 Piece Folding Table and Chairs Rustic Grey.
Part "K" in the manual is the brown piece with red button. See step 4 in manual for where it should be placed. The part contains is a locking pin for use when the table is in "open" (or unfolded) position. Use 4 Part "J" screws to attach it to table top's bracket. To use (after table is fully assembled), unfold table leaves, spin table top to open position, push silver cylindrical lock pin into corresponding hole on underside of table top. When finished and ready to fold the table top, push the red button on Part "K" (this releases lock pin), spin table top, and carefully fold down leaves.