Traversal means visiting every node in some defined order. The first three are depth-first and differ only by when you handle the current node relative to its children.
Pre-order: node first, then left subtree, then right. Good for copying a tree or writing a prefix expression.
In-order: left, node, right. On a binary search tree this yields sorted values.
Post-order: left, right, then node. Good for deleting a tree or evaluating results bottom-up.
Level-order is different: it visits nodes row by row from the top down, using a queue. That answers questions about distance from the root, like the shallowest level where something appears.
All four touch every node once, so each costs O(n). The choice is about order, not speed. Pick the one whose visit timing matches what you need to compute.
Rewriting in plainer words…
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