What it means
がる describes outward signs that someone has a feeling or reaction, such as fear, anxiety, dislike, or discomfort. It normally attaches to the stem of certain i-adjectives or na-adjectives and is most often used for another person or animal rather than for the speaker's own feelings. がっている describes the signs as an ongoing state, while the polite counterpart is がっています.
Usage note
がる is not freely usable with every adjective. It is especially common with adjectives describing emotions, preferences, physical sensations, or reactions. For a speaker's own feelings, use the adjective directly in most situations instead of がる.
How to use it
私の弟は試験の結果を不安がっています。
My younger brother appears anxious about the exam results.
その子は暗い道を怖がっています。
That child appears to be afraid of the dark road.
私の弟は試験の結果を不安がっている。
My younger brother appears anxious about the exam results.
その子は暗い道を怖がっている。
That child appears to be afraid of the dark road.