Delta is the option's sensitivity to a $1 move in the underlying. A 0.40 delta call gains roughly $0.40 if the stock rises $1. Delta is also a rough proxy for probability of finishing in-the-money: a 0.30 delta option has approximately a 30% chance of being ITM at expiration. That second meaning is the more useful one for strategy selection.
Theta is the option's daily premium decay. A position with -0.05 theta loses $5 every day the underlying does nothing (per contract). Long options have negative theta — time hurts you. Short options have positive theta — time pays you. That single distinction tells you why selling premium and buying premium are fundamentally different psychological games. Everything else is detail.