There are various ways to search in Slack. Here are some examples:
Use the searchbox at the top of the app or website:
The search item is to the left of the = and in bold for illustration purposes. The explanation of the search is to the right of the =.
Meeting = will search for messages with the word Meeting in them (also returns "meetings", etc.)
"Meeting" = messages with only the word Meeting
Meet* = messages that start with "Meet" (e.g. Meet, Meeting, Meetings, etc.)
in:staff Meeting = messages containing the word Meeting in #staff (in staff channel)
Meeting -Zoom = messages containing "Meeting" but not "Zoom"
* CTRL+F (⌘+f on macOS) pre-populate Search field with that modifier and you can add words to get the results you seek
to:@chrisc = messages directed to Chris
from:@chrisc = messages from Chris
dm:@chrisc = direct messages from Chris
has:star = starred messages
has:link = messages with a link in them
has::toiletpaper= messages with tp emoji
* you can filter search results to remove those added by bots and integrations (like imported tweets or Google docs). You can also limit searches to just channels visible in your channel list, or all channels, etc.
on:today (searches only messages posted today)
before:April (searches everything, you guessed it, before April of the current year)
during:week (searches only since the previous Sunday)
after:2019 (search includes 2019 and beyond, ignores earlier messages)
type:pdf student = search for the word "student" only within messages containing uploaded PDFs
from:me has:reaction = messages you posted, that others reacted to