Multimodal means that your baby is using a few ways to explore, so they might look at an object, touch it and bring it to their mouth. This supports their cognitive development as your baby combines information from their senses to learn about the object.
Your baby will probably explore things using more than one of their senses and for longer when they are lying on their back or sitting, for instance, this could be when you are holding them to sit. Tummy time is great for developing strength and control, but time in other positions might be good to help your baby learn about the world too.
Reference:
Soska, K.C & Adolph, K.E. (2014) Postural position constrains multimodal object exploration in infants. Infancy 19(2) 138-161.