Char the corn: Place the ears directly on a hot grill or a dry cast-iron skillet over medium-high heat, turning occasionally until lightly blistered and tender, 8–10 minutes total.
Let the corn cool until easy to handle, 5–10 minutes.
While the corn cools, finely chop the jalapeño and red onion, and chop the cilantro.
Cut the kernels off the cobs: stand each cob upright in a bowl and slice downward to remove the kernels (you should have about 3 cups).
Combine the corn, jalapeño, red onion, and cilantro in a mixing bowl. Squeeze in the juice of the limes (about 3 tbsp) and add the kosher salt. Toss to coat evenly.
Rest the salsa for 15 minutes to let flavors meld. Taste and adjust salt or lime to preference. Serve with tortilla chips or spoon over tacos, grilled meats, or salads.