There are many websites from where we can learn languages, history and geography, although there are not so many that can help us understand and practice mathematics.
Here I leave you with some resources that may be useful to you.
Mathematical concepts explained with applications and videos. There are several free tools for learning the basics.
With an extensive collection of videos, many of them available in Spanish, mathtv.com offers the possibility of reviewing mathematical concepts of algebra, calculus, trigonometry, geometry, etc.
Available in Spanish and very useful for practicing multiplication tables.
Very easy to use and available in Spanish. You just have to choose the operation and the level of difficulty, the application will open in the middle of the screen waiting for a response from the user.
This is a game in which you will have to mark the operations that appear on the board as correct or false. The speed can be increased and each level has more complex operations, making it more difficult to prevent the operations from touching the top of the frame.
Wolfram Online Integrator
Although the famous WolframAlpha already shows the result of the integral, here in the sub-domain we can see some more details about this particular operation, such as different ways of showing the result and description of the variables used.
ElFueloEduca contains excellently rated content, with no distracting resources for children using the portal. A very clean aesthetic, although somewhat dated.
Mathematical tool that will help us solve problems by entering it in the text box and indicating what type of problem it is (algebra, trigonometry, calculus, etc).
In addition to finding applications to install on a computer, you can consult sheets with formulas for algebra, calculus, trigonometry and geometry for viewing online or in PDF, with examples and definitions.
The approach is quite didactic, aimed at an audience that is entering and wants to hone their math skills online. With games classified into categories, experiments and news of all kinds.
From the hand of e-Tutor comes this interesting application to represent graphically one or several graphs, interactively: Graphing Calculator. With the cursor you can drag the graph to the region that interests you, and zoom, virtually to infinity. There are other similar options, such as archimy.com, calc5.com, and fooplot.com, for making 2D and 3D graphics.