MongoDB 2 node cluster
Because MongoDB automatic failover works election system. When the election system wants to decide a new primary node, it needs a qualified majority from secondary nodes or arbiter.
If you want to run a replica set of just two nodes, you should use the arbiter. The arbiter node resource cheaper than the other nodes.
Check out PSA in the document.
Priority means;

The value of the member’s priority setting determines the member’s
priority in elections. The higher the number, the higher the priority.

Imagine, running with 3 nodes replica set, one of them going down, and the other two nodes hast to decide a new primary. When they doing that check out their priority.

The priority settings of replica set members affect both the timing
and the outcome of elections for primary. Higher-priority members are
more likely to call elections, and are more likely to win. Use this
setting to ensure that some members are more likely to become primary
and that others can never become primary.

Otherwise, you should take a look at this document.