Trouble understanding the difference between observed and covariate in the aov_car package

I am trying to run a mixed-effects ANOVA on a 2 (between) by 3 (within) study. I've been following much of this tutorial https://ademos.people.uic.edu/Chapter21.html#21_data_background: but I'm having trouble adding covariates like age, gender, personality.

Looking at the aov_car documentation (https://github.com/singmann/afex/blob/master/examples/examples.aov_car.R), it seems that observed is used to represent covariates and thus, am having some trouble understanding the difference.

Can anyone clarify the difference between the observed and covariate inputs and if possible, let me know of the proper syntax for running the aov_car with covariates.

Note I'm somewhat new to R/statistics.