I had nearly finished my studies and had an idea to start a small software company, filling a gap in the market that was overlooked by the larger IT companies.

filling 是动词 fill 的现在分词形式，fill 的意思是"填充、填补"。 a 是不定冠词，表示"一个"或"某种"。 gap 的意思是"缺口、空白"，指未被满足的市场需求或未被开发的领域。 filling a gap 这个短语的意思是"填补空白"，在商业语境中特指发现并满足市场上未被满足的需求。