Moderate tension (40 to 50 lbs.) offers more comfort, spin and power. The flip side is that you'll have less control and accuracy.
High tension (>50 lbs.) provides greater control and accuracy. It can, however, lead to tendinitis. You will also have less spin and power.

For monofilament strings, Decathlon recommends a tension of between 42 and 49 lbs. We strongly recommend against using monofilament strings for children under 15 to avoid injury. These strings are best for players who break strings in under 10 hours of play because they quickly lose their tension.

For multifilament strings, Decathlon recommends a maximum tension of 55 lbs. These strings are recommended for people who are prone to tendinitis, and youngsters. Multifilament strings keep their tension very well.

Your racket's performance depends 50% on your frame and 50% on your strings! Be sure to take the time to choose the right strings.
For players who break their strings a lot: change them as soon as they break.
For people who rarely or never break strings, Decathlon recommends changing them as many times per year as you play per week. E.g. if you play once per week, change your strings once per year. If you play 3 times per week, change your strings 3 times per year.

You shouldn't tighten strings that have gone dead since their performance will have been lost. A loosely strung racket will let you play a more relaxed game, giving you more power without having to force it. Over time, strings can become worn without breaking. They lose their elasticity and initial properties. The gauge (or diameter) also affects play. The smaller the gauge, the more power and spin you'll get. A larger gauge will give you stiffer strings with a heavier impact on your body.