There are many competitions for table tennis, but the ones which can be very challenging are the international ones, for example the ITTF Opens. There are both senior and junior world tour events held all over the world. I regularly compete in the ITTF Junior events and then also compete in the regional championships, such as the European Youth Championships.
When you play tournaments in your home country, you often know the other players as you will have competed against them many times and you know the system for the tournament, how the draw is done and who you play when etc. The travel might be as simple as a short car journey from your home. When you play abroad it is very different environment as you may never have faced these players before and the actual competition might be set up differently, matches could be spread over several days. This takes some getting used to, everything from being able to practice before matches to finding your way around the venue. It is a big logistical challenge because you have to plan the travel to and from the country and then within the city you are playing. All this is on top of getting ready to play at your best. There can be several formats of competition, from the singles and doubles to the team matches. This means you have to be ready to play across a variety of circumstances.
The other consideration is your ranking for international events is totally different to your ranking in a country. So in order to have a world ranking you need to compete in the international tournaments and of course you start at the bottom with zero points. So for your first tournament you start with a tough draw. The more you compete and gain points, gradually your ranking will improve and your seeding should improve also. The ITTF now use a system which is based on progression in a tournament, you don't "lose" points if you lose a match.