before sunset
Refers to the time period just before the sun goes down. Often used in weather forecasts, schedules, or descriptions of evening activities.
Please try to get home before sunset.
The sky before sunset is beautiful.
夕方 is a more general term for 'evening' or 'late afternoon', whereas 日没前 is more precise about the time just before the sun sets.
Compound of 日没 (sunset) + 前 (before). The reading is the standard on'yomi for each kanji.