Terms Defined 
Let's define a few terms. Phonology is pattern of speech sounds, while articulation is how the sounds are made. When children are learning speech and language, they also have to learn the rules (phonology) as well as how to form the sound in their mouths (articulation). Each has their own age-range that kids typically figure things out and therefore master the sounds and rules.
Both phonology and articulation have errors associated with each of them, and they are very different. This was probably the most confusing thing to me when I was learning about it in graduate school, but after I figured it out, it made so much sense!
Errors Defined
Phonological errors are very common in young childhood when kids are learning the rules of the sounds. A phonological error might sound like "wabbit" for "rabbit," or "dod" for "dog," in which a child breaks the rules of how our sounds go together. I found this table that I love--it lists all of the phonological processes (or ways we see rules broken, which examples) and the ages for which children usually figure them out and resolve them.
An articulation error is an error in how one forms the sound with the mouth. It might sound like a lisp, in which the tongue comes out between the teeth for the /s/ sound, or the /r/ sound is distorted, often it is a tongue placement error or an airflow error. These are often very distracting to the listener. This table shows the age range for when you can expect a child to master each sound's placement and/or airflow.
When to Worry
Whenever a phonological process or articulation error is 1 year past the developmental range, it is considered an error. Example 1: So let's say that your little one is 4 years old, and saying "share" for "chair." This is not an error yet because this error pattern usually resolves around age 4, so it wouldn't be considered an error until the 5th birthday. Example 2: Your little one is 7 years old and the /r/ sound is muddled a bit. This is an error because the /r/ sound is expected to be accurate by the age of 6, so this child is one year past the age we would expect for the /r/ sound to be produced correctly.
A lot of the errors I see in children are, in fact, phonological errors. *food for thought* Phonological errors often coexist with reading problems because the child has trouble learning the rules, which often apply to sound-letter correspondence (pairing a sound with a letter, which is an early stage of reading).
So what do I do?
A Speech-Language Pathologist (CCC-SLP) can evaluate your child's speech with a simple picture-naming test. The test is quick and if your child's errors are outside of the developmental range (one year past the typical age of mastery for that sound or phonological process), he/she may recommend speech therapy.
At least in Tennessee, the state provides parents free resources depending on the child's age. Tennessee Early Intervention System (TEIS) is a free program that serves qualifying children from birth until the 3rd birthday. Or if your child is 3 years or older, they may be eligible for services through your school system. Every state is different, so check with your pediatrician or state's website for information regarding services that may be available in your area.
Above all, know that it is completely normal for your child to make errors while they are learning to talk--it's a lot to learn! If you are concerned, reach out for help, and if your mommy red-flag keeps going up, keep asking! Eventually, you will either have your worries calmed, or get your child the help they need.