Minecraft – What does it mean when animals have hearts over there heads

LAN stands for Local Area Network. It is the same thing as local multiplayer servers, and you should use it when you want to play with your friends on the same wifi.

If you've built it underground, you'd have to lit all caves around, so the mobs won't have any other spawn possibility than inside your grinder. Your best option is to build it as high as possible (remember that on MCPE maximum construction layer is 128).

As an extra recommendation, I advise you build it over the far ocean. Usually, there are some islands near the shore. Choose one and, from its farthest (relative to the shore) boundary, count around 128 blocks towards the far ocean and then start climbing towards the sky as high as possible for your grinder to fit under 128.

With that distance, mobs won't have any other options to spawn but inside your grinder. Remember to stay around (but not too close, as you already did) for them to spawn.