开 (traditional 開) pictured a door (门) with two hands lifting the bar that bolts it — the very act of OPENING a gate. The simplified 开 keeps just the bar and the hands beneath.
开 is one of the busiest verbs in Chinese. OPEN: 开门 (open the door), 开口 (open your mouth = start to speak, with 口). TURN ON / START: 开车 (drive, 'start the vehicle'), 开会 (hold a meeting), 开始 (to begin). And the lovely 开心 (kāixīn, 'open-heart') = happy — when your heart opens, you're glad. 开学 (with 学) = school starts. Whenever something begins, switches on, or opens up, you reach for 开.
Two hands lifting the door-bar — to OPEN.
开心 ('open heart') means happy; its opposite, an unhappy heart, is 不开心. Chinese keeps emotions in the 心 (heart): open = glad, tight = anxious.