Before the pandemic, SNY had an actual informative segment on their Baseball Night in New York show. It featured former Mets catcher Anthony Recker. He talked about some of the sequences that the Mets used to prevent sign stealing back when he was on the team.
Depending on the inning, the Mets would use a different code. For example, innings 1-3, 4-6 and 7-9. Some pitchers would keep a card inside their hats to remind them of which code was being used for each inning. The card was especially useful for relievers coming into the game late.
The particular sign sequence demonstrated in this video with Anthony Recker is “chase the two”. The pitcher looks for what sign is given after the catcher puts down a two. Former Mets player Todd Zeile pretends to be a batter in the video.
What the video doesn’t show and which they discussed earlier is the ABE system. Where A is for ahead in the count, B is for behind in the count and E is for even in the count. The pitcher and catcher will use a different sign for each situation during an individual at-bat. For instance, maybe use the first sign when ahead in the count. The third sign when behind in the count and the second sign whenever the count is even.
Related: Jeff McNeil answers your questions.