When the man with the ball starts running, you run after him and tackle... Oh my bad, wrong tackle. Okay, what I meant to say was tackle as in the fishing equipment used during your expedition in the outdoors.
Tackle is basically the fake bait that you use to catch fish and the accessories used to help you catch them.
Your tackle is what you use to catch your fish. To determine what kind of bait you use out of your tackle, use the methods talked about in the "Environment" page. Such as looking under rocks, to see what kind of creatures live under there.