In order to join the Tangle, a new transaction must choose two previous ones to approve. In general, it will choose two previously unapproved transactions, ortips. The method for choosing these two tips is called the tip selection algorithm.
Nodes are not obligated to follow the recommended tip selection algorithm, which is outlined in this section. However it is designed so that if most nodes do follow it, the rest are incentivized to follow suit.
Tip selection is done by performing a weighted random walk from the genesis towards the tips. The walk stops when it reaches a tip. The walk is performed twice, and so two tips are chosen.
The walk is biased towards transactions with more cumulative weight, or more transactions referencing them. This creates an incentive to approve new transactions rather than old ones.