The free content is covered by different licenses, so this question cannot be answered by a 'yes' or a 'no'. You will need to use free content that are licensed for commercial purposes.
The license information for each item will be found next to each item, as shown in the screenshot below. There is a clickable link to the license terms. If the item cannot be used for commercial use, this will be clearly stated in the license terms or even the license name.
To make it easier for you to find items you can use, there is a filter for the License Type. With this filter, you can search only for content that is suitable for commercial use.
If you have any questions regarding licenses, please check our License overview article.