What hearsay means
Hearsay, loosely put, is an out-of-court statement offered to prove that what it says is true. The classic worry is a secondhand account: someone testifying about what another person told them, where the original speaker cannot be questioned. Evidence rules limit hearsay because the person who made the statement is not there to be tested.
The rules are genuinely technical and riddled with exceptions, and how they apply to a personal journal or a text message is not something you can safely predict on your own. This is one of the clearest reasons that what your record can be used for is a question for a lawyer, not a matter of common sense.
Example: your own contemporaneous notes and a screenshot of a message the other parent sent you may be treated very differently from your note about what a neighbor said they saw. Which pieces come in, and for what purpose, depends on the rules in your court.
The practical takeaway is not to try to master the doctrine but to keep clean, firsthand, dated records and let your attorney sort out how each piece can be used.