To strengthen something means to make something stronger or to reinforce something so it is able to withstand more pressure or force. To 'strengthen' is a verb used in both professional and social contexts.

1. The recent holiday my wife and I took has really helped to strengthen our marriage. It was exactly what we needed after a difficult year.
2. I have been taking a vitamin supplement in order to strengthen my hair and nails.

1. Alice's exemplary work recently has greatly strengthened her chances of being promoted. The boss has been really impressed.
2. The setbacks Jake has suffered so far this year have only served to strengthen his resolve to get his business up and running. He is extremely driven.

1. The accident only strengthens the claims of staff members who have long accused management of willfully presiding over an unsafe working environment in order to cut costs.