When people talk about Islamic influences on Christian art, they mean that artwork made by Christians started to look like artwork made by Muslims. This happened because Christians and Muslims lived very close to each other in the Middle East and North Africa during the Middle Ages, a long time ago.
Christian artists saw beautiful things that Muslim artists made, like colorful tiles, fancy calligraphy, and intricate patterns. They thought that those things were pretty and started to use similar ideas in their own art.
For example, Christian artists started to use geometrical designs in their paintings and buildings, just like Muslim artists did. They also started to use bright colors and patterns, like the ones they saw on Muslim rugs and carpets.
This mixing of two cultures is called cultural exchange, and it helped to make beautiful art that people still enjoy today.