The expression "Catch the localizer" refers to runway approaches with the autopilot engaged. The angle between the aircraft heading and localizer beam should be less than 30 degrees, and the indicated airspeed at least below 250 knots (for jet airliners), then by pushing a button marked "APP" or "ILS", then the autopilot presumably will turn and then follow the localizer. The autopilot will then also automatically descend according to the glideslope. Normal procedure is to capture the localizer first and then follow the glideslope as well. If the angle is too large or the airspeed too high, capturing the localizer may be unsuccessful.