今儿个 is a colloquial Northern Mandarin (especially Beijing dialect) word for 'today'. It's a rhotacized version of 今天 (jīntiān), with the 儿 suffix and an extra 个 that adds a casual, local flavor. You'll hear it in everyday conversation in Beijing and surrounding areas, but it's not used in formal writing or standard Mandarin textbooks. Think of it as the equivalent of 'today' in casual Beijing speech — friendly and informal.