To put or place something on top of another thing (colloquial), e.g., placing food on a plate or a book on a box.
サラダをパンに乗っけて食べた。
I put the salad on the bread and ate it.
荷物を箱の上に乗っけておいてください。
Please put the luggage on top of the box.
写真をアルバムの最初のページに乗っけた。
I placed the photo on the first page of the album.
To give someone a ride or pick someone up (colloquial), a casual form of 乗せる meaning to transport someone in a vehicle.
駅まで乗っけてくれる？荷物が多くて助かるんだ。
Can you give me a ride to the station? I have a lot of luggage and it'd help.
友達を家まで乗っけてあげた。
I gave my friend a ride home.
古いラジオをトラックに乗っけて運んだ。
I loaded the old radio onto the truck and transported it.