The term "secure" is up to interpretation. Whether or not something is secure depends on what risks you are trying to mitigate.
Let's use the example of bleachbit, mentioned in another answer. Since bleachbit overwrites data, it's extremely unlikely to recover any files explicitly erased this way.
However it's still possible that someone would be able to recover information you deleted with bleachbit. Let's say you used bleachbit to securely erase a spreadsheet file with personal and financial information. While that particular file may be unrecoverable, there might be other copies of that file you aren't necessarily aware of, like backup files saved by the software you used to author and edit that file.
If you are trying to secure personal information from the potential risk that someone obtains physical access to your device, it is best to use full disk encryption. Full disk encryption ensures that even if someone gets access to your device, they will not be able to access any data without the decryption information. The best time to set up full disk encryption is during system installation.
There is still the risk that someone could recover files that were deleted before you started using bleachbit or before you started using full disk encryption. That's because typical file deletion and even formatting a whole disk does not actually delete data, but instead marks the space as available. To protect against these risks, you will need to securely erase the entire drive. SSDs usually have a secure erase function which quickly and securely erases the whole disk. HDDs and SSDs without secure erase will need to have all sectors completely overwritten.
