It can be used in several ways:
* Physically strong: This is the most literal meaning. You could use it to compliment someone for lifting heavy weights, running a marathon, or having incredible athletic abilities.
* Mentally strong: It can also refer to emotional resilience, determination, or the ability to overcome challenges.
* Strong-willed: It can be used to acknowledge someone's strong personality, their ability to stand up for themselves, or their unwavering convictions.
The expression "très forte" emphasizes the high level of strength you are acknowledging.