The number of the event you are swimming in. Events can be broken into heats - if you have a letter next to the event number (ex. 3A-_-_), then you are in the second or later heat of the event (no letter = first heat, A = second heat, B = third heat, etc). The event order is listed above (and in the lineup), so you can figure out what stroke and distance you are swimming based on the event number.
Lanes are not the same as at practice: The numbers start at the middle and increase outward (so each team has a lane 1, 2, etc). The picture below is what the lane layout looks like for meets at FHHS.
If you have 3 numbers, you are in a relay. The 3rd number indicates which swimmer you are in the relay. For 100 yard relays, swimmers 1 and 3 start at the deep end, swimmers 2 and 4 start at the shallow end. For medley relays, the stroke order is: 1) Back, 2) Breast, 3) Fly, 4) Free. For 200 yard relays, all swimmers start in the deep end.
* If you do not have a third number, you are in an individual event, not a relay
You are swimming in Event 4, in lane 2 (second lane from the center). There is no third number, so it is an individual event. You are starting at the deep (diving) end.
You are swimming in Event 15, in lane 3 (outer lane of the pool). You are the second swimmer in your relay, so you are starting at the shallow end. Since Event 15 happens to be a medley relay, you will be doing breaststroke.
You are swimming in the second heat (Heat A) of Event 3, in lane 1 (closest to the center of the pool). This event is a relay, and you will be the fourth (final) swimmer in this relay - therefore, you will start at the shallow end.