AssumeUTXO is a feature that will make it and easier to run a node.

When you run a node it will first download the entire blockchain and validate every transaction which takes a few hours/days.

In this process your node will keep track of all bitcoins that are spendable. It stores this data in the utxo-set. Utxo is short for unspent transaction output.

The wallet in bitcoin core is only fully useable once this initial blockdownload process is completed. You cannot validate an incoming payment if you don't know the utxo set.

AssumeUTXO allows the node to request the full utxo set immediately from your peers. This allows users to receive payments without having to wait for the initial block download.

However, the security concept is slightly different