• 拡大かくだい — enlargement, magnification
• 拡張かくちょう — expansion, extension
• 拡声器かくせいき — loudspeaker, megaphone
• 拡散かくさん — diffusion, spread
• 拡充かくじゅう — expansion (esp. in capability or function)

拡 combines the radical 扌 ("hand") with the phonetic component 広 (wide), indicating the idea of using the hand to make something wider or to extend it — i.e., “to expand.”

サービス範囲はんいを 拡大かくだいする予定よていです。
We plan to expand the service area.

都市としの拡張かくちょうは 止とまらない。
The expansion of the city continues unabated.

カク is the standard on-reading for 拡, appearing in many compound terms. There is no commonly used kun-reading, and the character is almost always used in jukugo (compounds).