It means "There is no might nor power except by Allah," an affirmation that all ability to avoid harm and to do good comes only from Him. The Prophet (peace be upon him) said to Abu Musa al-Ash'ari, "Shall I not guide you to a word that is one of the treasures of Paradise? La hawla wa la quwwata illa billah." It is a phrase of complete reliance on Allah and is recommended in many situations, including in response to the mu'adhdhin when he says "Hayya 'ala al-salah."
Evidence — the daleel
Sahih al-Bukhari 6384 ṢaḥīḥSahih Muslim 2704 Ṣaḥīḥ
On deen2u
Informational, not a personal fatwa. Consult a qualified scholar for rulings on your situation.