What does the strong believer hadith mean?

Mostly, it means strength of faith, character and willpower. A believer who is strong in these is better in Allah's sight than one who is weak, and the hadith can include a healthy body too. It also says there is good in every believer, so no one should feel written off.

The full hadith

Abu Hurairah reported that the Prophet ﷺ said: "A strong believer is better and is more lovable to Allah than a weak believer, and there is good in everyone. Cherish that which gives you benefit, seek help from Allah and do not lose heart. If anything comes to you, don't say, 'If I had not done that, it would not have happened,' but say, 'Allah did what He had ordained to do,' for 'if' opens the gate for Satan" (Sahih Muslim 2664).

Most people quote only the first sentence. The rest tells you what to do with it: aim for what benefits you, rely on Allah and keep going.

What kinds of strength?

Faith and resolve: sticking to prayer, honesty and good deeds when it's hard. Many scholars see this as the main meaning.
Self-control: the Prophet ﷺ said, "The strong is not the one who overcomes the people by his strength, but the strong is the one who controls himself while in anger" (Sahih al-Bukhari 6114).
Knowledge and character: the Quran says of King Talut that Allah "blessed him with knowledge and stature" (Quran 2:247).
Physical strength: a healthy body helps you pray, work and care for others. See Does Islam encourage exercise?
Strength to do your job well: the Quran describes the best worker as strong and trustworthy.

إِنَّ خَيْرَ مَنِ ٱسْتَـْٔجَرْتَ ٱلْقَوِىُّ ٱلْأَمِينُ "The best man for employment is definitely the strong and trustworthy [one]." Quran 28:26

"There is good in both"

The hadith is not an insult to people who are weaker. It says there is good in everyone. Someone who is ill, disabled, grieving, anxious or new to Islam is still a believer, still loved by Allah and still able to earn reward. Strength is something to grow, not a test of whether you count.

The Prophet ﷺ himself had days of tiredness and hardship. Sometimes strength means asking for help, resting or getting treatment. See Can I have depression and still have strong iman?

What about "if only"?

The last lines are about how we handle setbacks. Do what you can, then accept what happens. Planning for the future is fine, but replaying "if only I had..." leads to regret and doubt. Say "Qadarallah wa ma shaa fa'al," meaning "Allah decreed it and what He willed He did." That is not giving up. It is the strength of calm acceptance after real effort. See What does "Qadarallah" mean?

How to build this strength

Pray on time, even when you're tired.
Learn something about your faith each week.
Move your body: walk, swim or train in a way you enjoy.
Pause before reacting when you're angry.

Key points

Strength here means faith and resolve first, and the body too.
Self-control in anger is called real strength (Sahih al-Bukhari 6114).
There is good in every believer, including the weak.
Do your best, rely on Allah and avoid dwelling on "if only."

Tip: Choose one small strength to train this month, such as praying Fajr on time or a ten-minute daily walk. Small steady habits build real strength.