The word “that” is often unnecessary, but not always. Be sure you don’t cause readers to stumble.
“Get rid of ‘that’; you don’t need it” is common advice, but as is often true with nuggets of wisdom about writing, it fails to take into account those occasions where that is necessary to ensure clarity.
Consider this sentence: While riding the train, it is hard not to notice most people are leaning into a digital screen. As you read through the sentence, it appears to be saying that “it is hard not to notice most people,” but obviously, that isn’t the meaning of the sentence. When you have a verb (in this case, notice), look at the first noun or pronoun that follows it (people). Is that what the verb is referring to?
In the sentence above, it appears initially that people is the direct object of notice, but the real object is the clause that most people are leaning into a digital screen, so that is needed for clarity. Look closely at your sentence. If you see that the first noun or pronoun following the verb is not the appropriate object, keep that in the sentence (definition: The direct object is whatever is affected by the action of the verb).
In other sentences, that might not be needed, though you can use it if you want to. A sentence such as The folder that you need is on my desk could be written The folder you need is on my desk with no impact on clarity. Similarly, instead of saying, “Our office is in a tall building that is made of brick,” most people would say, “Our office is in the tall, brick building.” By reducing the clause “that is made of brick” to the adjective “brick,” you save words.