Nouns ending in -elle are usually feminine.
The suffix often forms a diminutive feminine.
Is belle masculine or feminine?
labelle
The masculine form is beau
The rule behind it
95.1% of -elle nouns are feminine
In context
C'est vraiment une belle journée aujourd'hui.
It is truly a nice day today.
Les gens disent que Nancy est très belle.
I hear that Nancy is very pretty.
Merci de m'avoir envoyé une belle lettre.
Thank you for sending me a nice card.