The Discovery & Experiment cards are great tools to frame your learnings objectives. According to your situation, you will use one or the other. Most of the time, your will start with Discovery Card(s) and continue with Experiment Card(s).
Use the Discovery Card to understand better who your customer is, which are his/her pain points, and identify opportunities to delight.
Use the Experiment card to frame how you want to test your idea/solution. Define learning goal and related hypothesis, select the relevant metrics to (in)validate this hypothesis, and document the action plan for gathering those metrics.