For tracking your food, you can use the Track by Nutritionix skill, which lets you record your food intake using your voice, or ask for caloric values of foods. (Alexa does the latter by default.) Say things like, "Alexa, tell Food Tracker to log a cup of almond milk" or "Alexa, ask Food Tracker how many calories are in two eggs and three slices of bacon."
Alexa Skills themselves are quite helpful. However, even with an updated Skills section in the Alexa app and the ability to add skills using only your voice, discovering new and useful skills is a less than desirable experience. So much so that Amazon actually created a skill called Skill Finder which, you guessed it, helps you discover new skills. Launch it by saying, "Alexa, open Skill Finder" or "Alexa, tell Skill Finder to give me the skill of the day."
Some Alexa Skills connect to smart devices, bringing voice control to a smart home from smart lights to locks, smart thermostats to televisions and more. Alexa becomes a home's second in command: users tell Alexa to tell other devices what to do. There are Alexa Skills for almost any kind of smart home device. These Alexa Smart Home articles can get users started immediately, quick start guides for using Alexa: 8 Alexa skills for your professionally installed and monitored security systems, 5 Alexa skills for security you install yourself, but have professionally monitored, and Alexa Skills that help lock down the security of your smart home.
Like all AI devices that use National Language Processing or NLP, Alexa does not understand all voices easily. But she learns to understand her people over time. She does, however, require people to speak in simple terms, with appropriate pauses, and use specific word orders. There are many times when she will say "I'm sorry, I don't know the answer to that question." More than likely, she doesn't recognize a word order or misunderstood the question.